Copper sulfide ores and the accompanying silicate ores often contain gold. There are two main processes for recovering gold from such ores: One is to treat copper sulfide or silicate ores containing gold (herein after referred to as “material”) at high temperatures above 1000° C. The material is melt with iron sulfide, or silicate ores for making up for the shortfall at such elevated temperatures to form Cu2S called matte and slag which is primarily composed of iron oxides or silicates and contains impurities. The matte is reduced at elevated temperatures to low-purity metallic copper called blister copper, which is refined by electrolysis into 99.99% or higher purity of metallic copper. Gold contained in the material behaves together with copper in the course of production of the metallic copper, and is recovered as a deposit, called copper electrolytic slime, together with other precious metals in the course of electrolysis.
The copper electrolytic slime is melt with lead at elevated temperatures, followed by concentration of the precious metals in the lead and removal of the lead through oxidation, resulting in a silver plate containing precious metals such as gold, called primary silver plate. The primary silver plate is electrolyzed to refine silver, in the course of which a deposit containing precious metals generates as silver electrolytic slime. From this deposit, precious metals other than gold are eluted with, for example, nitric acid, and then are melt at elevated temperatures to provide low-purity gold called primary gold plate, which in turn is subject to electrolysis to recover high purity gold.
However, this process has disadvantages of need for elevated temperatures above 1000° C., and substantively many steps to recover gold produced as a by-product in the process suited for production of metallic copper.
In the other process of recovering gold, the material is put into contact with a solution containing a complexing agent, which can readily form a gold compound, such as a cyanides, thiourea, or thiosulfuric acid, to allow such a agent react with react gold. The gold is eluted into the solution, and recovered through adsorption onto activated carbon.
Although this process can be free from any elevated temperatures and reduce the recovery steps, part of the agent for forming a gold compound will be consumed by other concomitant metallic elements such as copper and iron. Therefore, these other metallic elements must be removed to appropriate levels. In this case, additional steps of removing elements other than gold and recovering gold should be conducted (see “REVIEW OF GOLD EXTRACTION FROM ORES”, S. R. La BROOY, H. G. LINGE and G. S. Walker, Minerals Engineering, Vol. 7, No. 10, pp. 1213-1241).
Another example is to conduct gold leaching in an aqueous halogen-containing solution by utilizing a tendency for gold to form soluble complexes with halogen elements to dissolve in the aqueous solution. In this case, oxidizing agents must be added for ionization of gold. Such oxidizing agents that can be used typically include those having a standard oxidation-reduction potential of over +900 mV, such as nitric acid, hydrogen peroxide, and chlorine. Under conditions containing such oxidizing agents, most metallic elements that are contained may be oxidized with the agents, which leads to consumption of excess amounts of oxidizing agents than that necessary for recovering gold (see “Dou Chindenbutsu Shissiki Shorigijutsu no Kakuritsu (Development of Hydrometallurgical Process of Copper Anode Slimes)”, Akinori Toraiwa and Yoshifumi Abe, Shigen-to-Sozai, Vol. 116, (2000), No. 6, pp. 484-492).
Known as a so-called INTEC process, a leaching process has been proposed that involves generating elemental sulfur at a normal pressure, without cyanidation (Japanese Patent No. 2857930). The leaching solution that is used for gold leaching in this process has experienced copper electrowinning in a chloride bath for recovering copper. Such a solution has an oxidation-reduction potential of 700 mV, and leaches gold by higher oxidation-reduction potentials of halogen compounds (Halex: typically, BrCl2−) in the solution. Copper electrowinning in the chloride bath proceeds according to the following reactions:At the cathode: 2Cu++2e−→2CuAt the anode: Br−+2Cl−→BrCl2−+2e−
In gold leaching with Halex, the Halex is produced in a vessel used for electrodeposition recovery of copper, which requires complicated operations. Therefore, this process has a disadvantage of troublesome handling, and is not suitable for large-scale leaching.
AU Patent Application No. 2003287781 “Recovering Metals from Sulfidic Materials” discloses a process of leaching gold in a chloride bath, which requires special oxidizing agents such as gaseous oxygen and chlorine in order to improve the gold leaching rate.
Japanese Unexamined Patent Application Publication No. 2005-298850 discloses a process of leaching gold out of leach residues through a hydrometallurgical process for copper where these leach residues are processed with a solution containing ferric ions and thioureas. Gold leaching into the solution not only from such leach residues but also from a material allows copper and iron to consume some agents for eluting gold. Such agents for eluting gold into the solution, such as cyanides, thioureas, and thiosulfuric acid, are typically expensive, and copper and iron that consume the agents are present together with gold in higher amounts compared to the amount of gold, which leads to higher cost for gold recovery.
The applicant proposed the following process for gold leaching in Japanese Patent Application No. 2007-086983 (not published yet) filed on Mar. 29, 2007 which claims priority to Japanese Patent Application No. 2006-264423 filed on Sep. 28, 2006.
This is “a process of leaching gold comprising leaching copper from copper sulfide ore that contains gold or contains silicate ore containing gold until the copper grade in the copper sulfide ore is reduced to 7.9% or less; mixing the resulting copper sulfide ore having a copper grade of 7.9% or less with a solution containing chloride ions and ferric ions or with a solution containing chloride ions and iron ions in such a state that oxygen in the air bubbled into the solution oxidizes iron ions into trivalent ferric ions; adjusting the pH of the solution to 1.9 or less with stirring; and leaching at least gold into the solution by the oxidative activity of the ferric ions.”
In the process in Japanese Patent Application No. 2007-086983, conventional processes of gold leaching are evaluated as follows.
(1) Such conventional processes consume large amounts of expensive agents.
(2) As a result of elution of copper and iron present in the form of sulfides, sulfur remaining on the surface of the material passivates the material, which may preclude the reaction. For this reason, copper and iron present are preliminarily eluted into the solution to reduce the contents of these metals prior to the gold leaching operation in order to reduce the amounts of the gold leaching agents to be consumed. Alternatively, some attempts have been made to avoid these problems by carrying out operations to remove sulfur from the material, such as roasting, in advance. Such operations, including complicated steps, cause high gold recovery cost and yield products from solutions of high acid concentrations. Aqueous solutions of such products are highly acidic. Since gold leaching operations are carried out in the alkaline pH range, the highly acidic solution must be neutralized prior to gold leaching operations. Agents used for neutralizing cause higher recovery cost.
(3) In addition, agents used, such as cyanides, thioureas, and thiosulfuric acids are expensive, readily degradable, and toxic, which requires rigorous steps during or after handling of the agents. This also cause higher process cost. In addition, a long reaction time leads to a long overall process time to the final product and increased gold holdup.
(4) Processes of leaching gold using halides and their gases such as chlorine and bromine have also been contemplated in that these reaction rates are higher than those for cyanides. In this case, use of halogen gases as oxidizing agents provides solutions in high oxidation states (in a chloride solution, the hydrogen standard potential of 1242 mV for gaseous chlorine, and 1070 mV for gaseous bromine), so that gold leaching can be accelerated. These processes also have disadvantages of use of expensive and strongly corrosive halogen gases, and thus difficult handling. Moreover, due to their high ionization potentials through the reaction, excess amounts of halogens cause the surface of the material to be passivated, which may inhibit the reaction from proceeding. Sometimes, iodine is used. In this case, iodine is consumed more than necessary by the reaction with iron contained as a gangue component in the material. In these processes, since such agents have very high oxidation potentials, oxygen is not an effective oxidizing agent. Even when these agents are reduced during the leaching reaction, oxygen in the air fails to oxidize their reduced forms, so that they cannot be reused in a simple way. The process disclosed in Japanese Unexamined Patent Application Publication No. 2005-298850 can solve these problems.